Composite oxides containing Li, Ni, Mn, and Co have recently been receiving attention as positive electrode active materials for non-aqueous electrolyte secondary batteries.
For example, Patent Document 1 proposes a lithium nickel manganese cobalt composite oxide having a volume resistivity under a compression of 40 MPa of 5×105 Ω·cm or less and a C/S ratio of 0.025 or less wherein C represents the concentration (% by weight) of carbon contained therein and S represents the BET specific surface area (m2/g). The use of a positive electrode active material having a volume resistivity equal to or lower than a predetermined value and a significantly low carbon concentration permits an improvement in battery performance. Patent Documents 2 and 3 also propose a composite oxide having a volume resistivity under a compression of 40 MPa of 5×105 Ω·cm or less.
In Patent Documents 1 to 3, composite oxides are prepared by spray drying. In the case of composite oxides prepared by spray drying, it is difficult to heighten the pressed density thereof, and it is thus difficult to heighten the capacity of the battery. Increasing the Ni content can provide a high capacity positive electrode active material. However, when the Ni content becomes high, the volume resistivity of the positive electrode active material becomes excessively low. In particular, when the volume resistivity is less than 100 Ω·cm, the battery safety is significantly impaired. On the other hand, if the volume resistivity becomes excessively high, the electronic conductivity of the composite oxide becomes low and the reactivity of the electrode decreases, so that the cycle characteristics degrade.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-340186    Patent Document 2: Japanese Laid-Open Patent Publication No. 2006-172753    Patent Document 3: Japanese Laid-Open Patent Publication No. 2006-253119